modeling the benefits of lid stormwater techniques for developable parcels in the nashoba brook...
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Modeling the Benefits of LID Stormwater Techniques for Developable Parcels in the Nashoba Brook Subwatershed
Bob Hartzel, CLM, CPESCSenior Water Resources Scientist
Project funded through:
Provide planning to aid 5 subwatershed towns in promoting LID as part of master planning, project permitting and development of bylaws.
Organization for the Assabet River
Project Overview / Goals
Project Overview / Goals
Develop model to evaluate the suitability and priority of developable parcels in the Nashoba Brook subwatershed for LID stormwater techniques.
Estimate the benefits and costs of LID stormwater management compared to “conventional” stormwater management.
Vine Brook
Nashoba Brook
Low Impact Development (LID)
An ecosystem-based approach to land development and stormwater management.
Goal: Mimic pre-development site hydrology
Conventional Development
Centralized Pipe Centralized Pipe and Pond Controland Pond Control
LID Development
Disconnected Disconnected Decentralized Decentralized DistributedDistributed
Conservation Conservation Minimization Minimization Soil Amendments Soil Amendments Open Drainage Open Drainage Infiltration BMPsInfiltration BMPsVegetative BMPs Vegetative BMPs Rain Barrels Rain Barrels Pollution PreventionPollution Prevention
Multiple SystemsMultiple Systems
Residential LID Site
LID Stormwater Controls
Rain Garden Treatment Train Approach
Raingarden Cell
Storm Drain System
Raingarden Cell
Flow Path
Grass SwaleGrass Filter
Strip
Low Impact Development Stormwater Controls
Bioretention Raingardens/barrels
Porous surfaces Bioretention
RaingardensA bowl-shaped garden designed to
capture and absorb stormwater.
Bioretention Cell
Similar to raingarden, more highly engineered:
• underdrain/riser pipe
• gravel bed
• engineered soils
Street Edge Alternatives (SEA)
Functional Landscape
Reduced Impervious Area
98% Stormwater volume reduction for 2-year storm
“SEA” Street: Maximized space for filtration, recharge and landscape elements
Watershed Communities
Town % of watershed
Acton 43%
Westford 35%
Littleton 14%
Carlisle 5%
Concord 3%
Subwatershed Delineation
A. “Developable Parcel” Inventory / Map (Not Retrofits)
Unimproved lots > minimum lot size requirement
Municipally owned property that is for sale.
Parcels protected though open space preservation, deed restrictions, etc. = Not Developable
Any portion of a parcel within wetlands = Not Developable
Task 1: Evaluate Parcels Most Suitable for LID
Inventory developed with 5 town planning departments
Developable Parcels Map
198 Parcels
11% of land in watershed
Summary Statistics:
• Size Range: 0.3 – 150 acres
• 50% of land comprised by largest 8% of parcels
• 178 residential parcels
• 12 commercial parcels
• 8 industrial parcels
Task 1: Evaluate Parcels Most Suitable for LID
B. Develop Model to Rank Site Suitability / Priority
S = Soils
D = Distance to Receiving Water
B = Shallow Bedrock
Imp = Impervious Cover Analysis
PWS = Lands Critical to Public Water Supply Quality and Quantity
LID Priority Ranking = 2 (S) + 1 (D) + 1 (Imp) - 1 (PWS) – 1 (B)
“…with proper design and maintenance, LID stormwater techniques can be used successfully at many sites with less than ideal initial conditions.”
Soil Amendments
Underdrains
Steep Slope Designs (Terraces, etc.)
“Not Enough Space”
A newly planted bioretention cell installed in the Dudley Pond watershed (Wayland, MA).
Lake Shirley Bioretention Cell
Lake Shirley Bioretention Cell
Lake Shirley Bioretention Cell Lesson: Small is beautiful!
Terraced bio-infiltration system (Plymouth, MA)
Lesson:
Sloped areas can be put to work!
Soils
A: Sand, loamy sand or sandy loam soils. High infiltration rates!
B: Silt loam or loam. Moderate infiltration when fully wetted.
C: Sandy clay loam. Poor infiltration when thoroughly wetted.
D: Clay loam, silty clay loam, sandy clay, silty clay or clay. Highest runoff potential, very low infiltration when fully wetted.
Hydrologic Soil Groups
Shallow Bedrock
Impervious Cover Assessment
Total Subwatershed Imperviousness = 12.1%
Impervious Cover Assessment
Total Subwatershed Imperviousness = 12.1%
Range = 5.9% - 20.2%
Impervious Cover Assessment
Public Water Supply Protection
Public Water Supply Wells
Interim Wellhead Protection Areas
Zone 2 Wellhead Protection Areas
Water quantity
Drinking water quality concerns
Parcel Ranking
Parcel Ranking
Parcel Ranking…Wetlands
Next Steps: Evaluate the Relative Benefits of LID Techniques
A. Create “Standardized” Development Scenarios LID Conventional Development Cluster / Open Space Development (Residential parcels only)
B. Compare LID vs. Traditional Stormwater Mgt. Water Quality / Quantity Benefits Cost
Parcel Statistics: Choosing a “Representative” Parcel
Median (50th Percentile) Values: Size: 0.9 Acres % IMP: 8.4%
Examples: “Representative” Residential Parcels
Size: ~0.9 acres Imperviousness: ~8%
Examples: “Representative” Commercial/Industrial Parcels
Size: ~1.3 acres Imperviousness: ~30%
PRECIPITATION
STORAGE
INFILTRATION
RUNOFF
EVAPORATION
SWMM(VOLUME)
EVENT MEAN CONCENTRATION
(QUALITY)
LOAD = RUNOFF VOLUME X EMC
Quantifying LID Benefits: Infiltration and Load Reduction
Modeling Hydrologic Budget: EPA’s SWMM
INPUTS
Precipitation: 57 years of Boston rainfall data
Evaporation: data obtained from MassGIS
Subwatersheds based on surface types: roofs, pavement, lawn, wooded, etc.
Infiltration BMPs: drywells, bioinfiltration
OUTPUTS
Runoff totals from subwatersheds
Precipitation/Runoff statistics
Single Family Lot vs. Residential Subdivision
Parcels with 1-4 buildable lots = “Single Family Lots” (138 parcels, 285 ac.)
Parcels with 5+ buildable lots = “Residential Subdivisions” (40 parcels, 920 ac.)
Conceptual Single Family Lot: Conventional Development
Runoff: 8.9 in/yr Infiltration: 28.1 in/yr TSS: 213 lb/ac/yr TP: 0.72 lb/ac/yr
Conceptual Single Family Lot: Low Impact Development
Runoff: 1.9 in/yr Infiltration: 35.2 in/yr TSS: 68 lb/ac/yr TP: 0.27 lb/ac/yr
Runoff: -7.0 in/yr
Infiltration: +7.1 in/yr
TSS: -145 lb/ac/yr (-68%)
TP: -0.44 lb/ac/yr (-63%)
Conceptual Residential Subdivision: Conventional Development
Runoff: 11.8 in/yr Infiltration: 25.2 in/yr TSS: 330 lb/ac/yr TP: 0.92 lb/ac/yr
Conceptual Residential Subdivision: Low Impact Development
Runoff: 0.7 in/yr Infiltration: 36.5 in/yr TSS: 38.4 lb/ac/yr TP: 0.06 lb/ac/yr
Runoff: -11.1 in/yr
Infiltration: +11.3 in/yr
TSS: -292 lb/ac/yr (-88%)
TP: -0.86 lb/ac/yr (-93%)
Conceptual Commercial/Industrial: Conventional Development
Runoff: 16.3 in/yr Infiltration: 20.8 in/yr TSS: 415 lb/ac/yr TP: 1.73 lb/ac/yr
Conceptual Residential Subdivision: Low Impact Development
Runoff: 1.1 in/yr Infiltration: 36.5 in/yr TSS: 24 lb/ac/yr TP: 0.07 lb/ac/yr
Runoff: -15.2 in/yr
Infiltration: +15.7 in/yr
TSS: -391 lb/ac/yr (-94%)
TP: -1.66 lb/ac/yr (-96%)
LID Benefits: Infiltration
The added infiltration from LID being applied to developable parcels may be as high as:
… for the population of Acton, Littleton, and Westford (~51,500 people)
22 gal/person/day
0 5 10 15 20
Single Family Lot
Residential Subdivision
Commercial/Industrial
gal/person/day
Added Infiltration from LID
2 extra toilet flushes (1.6 gal each) per person per day…
… for the population of Acton, Littleton, and Westford (~51,500 people)
1 5-minute shower (10 gal) per person per day…
1 dishwasher load (8 gal) per person per day…
22 gal/person/day = approximately…
LID Benefits: Infiltration
LID Benefits: TSS
… entering Nashoba Brook
The reduction in weight of TSS from LID being applied to developable parcels may be as high as:
190 tons/year
0 50 100 150
Single Family Lot
Residential Subdivision
Commercial/Industrial
tons/yr
Reduction in TSS from LID
LID Benefits: TSS
140 cubic yards of sand/sediment… entering Nashoba Brook each year
190 tons = approximately…
LID Benefits: TP
… entering Nashoba Brook
The reduction in weight of TP from LID being applied to developable parcels may be as high as:
1190 lbs/year
0 400 800
Single Family Lot
Residential Subdivision
Commercial/Industrial
lbs/yr
Reduction in TP from LID
1190 lbs/year of TP = approximately…
….growing per year in Nashoba Brook (and tributary ponds)*assuming 1 lb P : 1026 lb wet Phytoplankton, density Phytoplankton ~ density H20
14,760 cu. ft.
of algae
COST: LID vs. Conventional Stormwater
Thank you for your time!
Conventional Design Low Impact Design
Units Unit Cost Qty. Total Qty. Total
LANDSCAPING
Item:
Tree Removal 1 ac $9,150 0.8 $7,320 0.3 $2,745
Clearing/Grubbing 1 ac $7,500 0.9 $6,750 0.38 $2,850
Grass Seed/Sod 1 sy $6 3900 $23,400 1328 $7,968
Native Trees/Shrubs 1,2 ea $30 0 $0 5 $150
Soil Amendment 2 cy $51 0 $0 12 $627
Raingarden/Bioretention 2,3 sf $11 0 $0 770 $8,470
INFRASTRUCTURE/CONVEYANCE/STORAGE
Item:
Standard Asphalt Driveway 2 sy $10 94 $940 0 $0
Porous Pavement Driveway 2,3 sy $108 0 $0 94 $10,152
Drywell - (including inst.) 2 ea $1,500 0 $0 1 $1,500
TOTAL $38,410 $34,462
-10.3%
Notes: Construction/Material costs only. Does not include maintenance or property values.
1. From PA DOT construction cost spreadsheet (ftp://ftp.dot.state.pa.us/public/Bureaus/design/Pub287/Pub%20287.pdf)
2. Geosyntec cost estimate data, past projects
3. "Evaluation of LID Best Management Practices (BMPs) Opportunities," Charles River Watershed Association
Single Family Home
Residential Subdivision Conventional Design Low Impact Design Units Unit Cost Qty. Total Qty. TotalLANDSCAPING Item:
Tree Removal 1 ac $9,150 0.8 $7,320 3.6 $32,940
Clearing/Grubbing 1 ac $7,500 0.9 $6,750 4.6 $34,500
Grass Seed/Sod 1 sy $6 46800 $280,800 15936 $95,616
Native Trees/Shrubs (for bioretention areas) 1,2 ea $30 0 $0 200 $6,000
Soil Amendment 2 cy $51 0 $0 148 $7,525
Raingarden/Bioretention 2,3 sf $11 0 $0 19100 $210,100 INFRASTRUCTURE/CONVEYANCE/STORAGE Item:
Standard Asphalt Driveway 2 sy $10 1130 $11,300 0 $0
Standard Asphalt Roadway 2 sy $20 4670 $93,400 3090 $61,800
Porous Pavement Driveway 2,3 sy $108 0 $0 1130 $122,040
Drywell - (including inst.) 2 ea $1,500 0 $0 12 $18,000
Catch Basins 1,2 ea $3,000 7 $21,000 0 $0
CPP Storm Drain Pipe 1 lf $75 1710 $128,250 180 $13,500
Vegetated Swale 2 sf $2 0 $0 1840 $3,680
Detention Pond 2,4 cy $15 2450 $36,750 0 $0
Outlet Structure 2 ea $1,500 1 $1,500 0 $0 TOTAL $587,070 $605,701 3.2%
Commercial / Industrial Conventional Design Low Impact Design
UnitsUnit Cost Qty. Total Qty. Total
LANDSCAPING Item: Tree Removal 1 ac $9,150 1.1 $10,065 0.8 $7,320 Clearing/Grubbing 1 ac $7,500 1.1 $8,250 0.8 $6,000 Grass Seed/Sod 1 sy $6 3390 $20,340 1940 $11,640 Native Trees/Shrubs (for bioretention areas) 1,2 ea $30 0 $0 24 $720 Soil Amendment 2 cy $51 0 $0 18 $916 Raingarden/Bioretention 2,3 sf $11 0 $0 1800 $19,800 INFRASTRUCTURE/CONVEYANCE/STORAGE Item: Standard Asphalt Parking 2 sy $10 1475 $14,750 1100 $11,000 Porous Pavement Parking 2,3 sy $108 0 $0 370 $39,960 Drywell - (including inst.) 2 ea $1,500 0 $0 1 $1,500 Catch Basins 1,2 ea $3,000 3 $9,000 0 $0 CPP Storm Drain Pipe 1 lf $75 325 $24,375 0 $0 Vegetated Swale 2 sf $2 0 $0 470 $940 Detention Pond 2,4 cy $15 340 $5,100 0 $0 Outlet Structure 2 ea $1,500 1 $1,500 0 $0 TOTAL $93,380 $99,796 6.9%
Costs: Literature Review Comparison